A structural adhesive is applied to a bonding between metal materials to exhibit an adhesive force equivalent to or more than that of the metal materials against a specific external force applied to the metal materials and is widely employed in the field of the automobile industry, the airplane industry, the can manufacturing industry, the electric industry and the electronic industry in place of or together with the conventional method of construction such as riveting, bolted joint, welding and the like.
For example, in case of assembling or bonding of a car body, for a heming fixation at an edge between an outer panel and an inner panel such as a door, a hood or a trunk lid, a periphery of the outer panel is heming-formed against a periphery of the inner panel directing toward the inside of the car body and adhered to each other and then, after applying a structural adhesive onto a periphery of the inner panel in a line, both peripheries are subjected to a spot welding for a watertight and airtight sealing, and thereby fixing the outer panel with the inner panel.
For the structural adhesive applied as mentioned above, there has been required a rust prevention. Especially in the automobile industry, automobiles exported to a cold district have been regulated with respect to anti-corrosion. That is, in a cold district, an anti-freezing agent such as a rock salt or calcium chloride is spread over the road in order to prevent freezing of the road, and as a result, a corrosion of a car body is highly accelerated to impair appearance and decrease a durability of the car. For this reason, a rust inhibiting pigment such as red lead or zinc chromate has been employed but there is a safety and sanitation problem of human and animals due to a toxicity of heavy metals (e.g. lead, chromium etc.).
For this kind of structural adhesive, there has hitherto already been developed a one-pack type thermo-setting adhesive prepared by combining a resin composition containing a variety of modified epoxy resin with a latent curing agent. There have been known, for example, those prepared by combining a specific urethane modified resin and glycidyl ether epoxy resin with a latent curing agent and optionally a rust inhibiting pigment, metal powder and the like (see Japanese Patent First Publication "Kokai" No. 06882/1985) and those prepared by combining a specific rubber modified epoxy resin and aromatic epoxy resin with a latent curing agent and electroconductive carbon black (see Japanese Patent First Publication "Kokai" No. 53387/1987). Although these adhesives are endowed with electroconductivity and expected to improve anti-corrosion property by forming electrocoating film (electrodeposition) in the subsequent procedure, the former resin employing metal powder such as aluminum has the disadvantage that a pinhole is produced by generation of hydrogen gas in the electrode reaction and that there is a problem of rust generation at an interface of the periphery of panels to which the adhesive is applied. The latter resin has also the disadvantage that the anti-corrosion property of the adhesive itself is not sufficient though no pinhole is produced.